AN2577 View Datasheet(PDF) - STMicroelectronics
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MFG CO.
'AN2577' PDF : 34 Pages View PDF
System design
3
System design
AN2577
There are four operating conditions that must be considered in the system design process:
steady state operation, acceleration and deceleration, startup, and exception handling. The
first two conditions covers a large majority of the operating time of the system but requires
only a small portion of the development time. Startup is more complex, while exception
processing tries to anticipate large numbers of exceedingly rare error conditions, which must
be handled competently without human intervention.
Since the eTPU is only a small slave processor associated with a powerful MCU central
processor, the system approach should be to entrust the steady state operation an much as
possible to the eTPU, with assistance in startup. The eTPU might help in error detection, but
since the exception handling may be complex, the proper systems approach is to use the
eTPU to detect anomalies, but correct the errors where possible in the CPU.
3.1
3.1.1
Steady state system design
There are two basic angle counting modes that can be supported by the eTPU. In the
periodic mode, the TCR2 can be programmed to maintain a real time representation of the
absolute angle of the wheel with respect to a reference position, while in the free running
mode it can track a continuously increasing angle from startup, rolling over as the TCR1
(time) counter does after reaching 0XFFFFFF (16777215) counts. The choice between
these modes depends on the desired mode of comparison of angle positions.
Periodic angle clock
If the angle clock software sets LAST at the end of a cycle, typically at 360 or 720 degrees,
the TCR2 counter will roll over to 0 at the tooth edge after LAST. This means that
comparison to the engine angle must be done as equals-only. The greater-or-equal
compare depends on the count rolling over after 0XFFFFFF. Greater-or-equal compares will
be satisfied for any number from 0 to 0X7FFFFF less than the programmed angle. This
means that an attempt to setup a compare in the future will not work properly if the future
projected angle rolls over the end angle. In other words, 0 degrees will always test less than
any other angle, including 715 degrees, even though with the periodic angle clock, it is 5
degrees in the future.
Engineers are understandably uneasy about using an equal-only compare. If a spark is
programmed to fire at 32 degrees, missing the angle under any exceptional condition will
cause an undesirable misfire. However, the eTPU angle clock is designed not to miss any
comparison counts regardless of the speed dynamics of the input signal. This means that
any compare that is programmed to occur at a selected angle will absolutely occur at the
next occurrence of that angle in TCR2.
The only danger is in writing the compare value just too late for the intended cycle. If the
spark time was changed, for example, to 31 degrees just after the angle clock reached 31.1
degrees, the spark could be missed. Therefore it must be a requirement of all angle
triggered driver software to test for any change that steps the compare point over the current
angle, and force the compare immediately when that happens.
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